1. Field of the Invention
The present invention relates to a laser apparatus which is employed in the optoelectronics field and, more particularly, to a laser apparatus employed for a laser printer, particle counter, optical inspection apparatus for medical use, photofabrication apparatus, and optical disk apparatus.
2. Description of the Related Art
With the growing advanced information age, there has been increasing demand for shorter wavelength in computer peripheral apparatuses incorporating laser such as an optical disk apparatus and laser printer in order to meet the needs for higher recording density and higher printing speed. There are currently available, however, only gas laser apparatuses including a helium-cadmium (He-Cd) laser apparatus and argon (Ar) laser apparatus as light sources capable of providing satisfactory blue radiation for which there is high demand at the commercialization level. These gas lasers are large and heavily consume electric power. Since the laser apparatuses are large, laser-applied apparatuses, which incorporate laser apparatuses as light sources thereof, must be at least as large as the laser apparatuses. Thus the apparatuses employing lasers unavoidably become large, presenting a problem in that they are hardly suited for use in office environment or residential environment where desktop size is dominant.
Further, such laser apparatuses have low efficiency of conversion from supplied power into laser output power; the majority of the power consumed turns into heat, leading to the need of a cooling means. This adds to the size of an apparatus incorporating a laser. There is another problem: an optical system is dislocated by vibration of the cooling means, deteriorating the reliability of the laser-applied apparatus. There is still another problem: the service life of such a laser apparatus is short mainly due to the deterioration in charged gas, resulting in a shortened life of the laser-applied apparatus.
An attempt has been made to solve the problems involved in the gas lasers stated above by applying a wavelength converting art typically represented by the second harmonic generation (hereinafter referred to simply as "SHG") in which a nonlinear optical crystal is inserted in a resonator of a solid-state laser so as to accomplish conversion into a wavelength which is half the wavelength of the first laser beam (hereinafter referred to as "fundamental wave" as necessary) which is the oscillation wave of the solid-state laser. For example, the SHG system for solid-state laser has been proposed which employs Ti:Al.sub.2 O.sub.3 (Ti-Sap;Ti-Sapphire) or Cr:LiSrAlF.sub.6 (chromium doped lithium strontium aluminum fluoride: hereinafter sometimes referred to simply as "LiSAF") which is a laser crystal capable of oscillating over a wavelength range of, for example, 800 to 900 nm. The use of such solid-state crystals instead of gases has led to a dramatically prolonged service life of laser medium unit. The pumping source of the LiSAF laser, however, is still a krypton (Kr) laser, i.e. a gas laser; therefore, the LiSAF laser still has the problems unsolved. ("Tunable blue light source by intracavity frequency doubling of a Cr-doped LiSrAlF.sub.6 laser", Appl. Phys. Lett., vol. 61, No. 20, p 2381 (1992) by F. Balembois, P. Georges, F. Salin, G. Roger, and A. Brun) Furthermore, the Ti-Sap laser employs, as its pumping source, a green SHG laser having a Q switch Nd:YAG (neodymium doped yttrium aluminum garnet) laser for its fundamental wave. The use of the solid-state design for the whole laser light source has achieved a prolonged service life; however, the problems stated above have not been considerably improved because the completed apparatus incorporates two SHG lasers including the pumping source, resulting in a complicated design, large size, and heavy power consumption. In either case, SHG outputs are in the form of pulses, posing problems such as one wherein signal discontinuity takes place in a laser-applied apparatus including a laser printer, photofabrication apparatus, and optical disk apparatus.
It has been disclosed that a red semiconductor laser having a wavelength of 670 nm can be used as the pumping source of the aforesaid LiSAF ("Diode-pumped Cr:LiSrAlF.sub.6 laser", Opt. Lett., Vol. 16, No. 11, p820 (1991) by R. Scheps, J. F. Myers, H. B. Serreze, A. Rosenberg, R. C. Morris, and M. Long). Thus, it is expected that there is a possibility of significantly improving the problems of the large size, heavy power consumption, and short service life of the conventional gas laser apparatus by replacing the Kr laser with a semiconductor laser for the pumping source of the SHG light source which employs the foregoing LiSAF laser for the fundamental wave.
It has been disclosed, however, that the LiSAF laser exhibits a markedly low efficiency due to the loss in the resonator ("Electronically tuned diode-laser-pumped Cr:LiSrAlF.sub.6 laser", Opt. Lett., Vol. 17, No. 1, p43 (1992) by Qi Zhang, G. J. Dixon, B. H. Chai and P. N. Kean). This means that the feasibility of practical utilization stays extremely low unless the cause of the low efficiency is identified and examined to find a specific solution.
Qi Zhang et al. took out a birefringent filter, which is a wavelength tuning element for enabling the LiSAF laser to select wavelength, from the resonator of the LiSAF laser. In general, the birefringent filter, which is provided in a resonator of a laser such as the LiSAF laser capable of wavelength tuning, is designed to give the loss typically represented by reflection loss to oscillation beams other than the oscillation beams having a desirable wavelength, thereby selecting laser beams of the desirable wavelength. Qi Zhang et al. added a resonator exclusively designed for wavelength tuning which includes the birefringent filter to the rear stage of the resonator of the LiSAF laser. More specifically, Qi Zhang et al. has proposed a means for feeding back the return beams of the desired wavelength to the resonator of the LiSAF laser and amplifying them so as to perform wavelength tuning without the loss caused by the resonator of the LiSAF laser (U.S. Pat. No. 5,218,610). The loss on which Qi Zhang et al. focused their attention was primarily the reflection loss caused by the birefringent filter. The method had a possibility of an apparatus equipped with the means becoming complicated and large although it was capable of reducing the loss.
Further, there is an example wherein a corrective measure has been taken with attention given to the fact that the uneven energy absorption of the LiSAF crystal, which is a laser crystal, adversely affects the efficiency of laser oscillation. The example shows an attempt to improve the efficiency by distributing Cr involved in the absorption in different concentrations in the exciting direction (U.S. Pat. No. 5,287,373). This method, however, has not yet been realized at a commercial level and it does not clearly indicate improvement in the problems identified by Qi Zhang et al.
In the optical system disclosed by F. Balembois et al. mentioned above, a Kr laser is used as the pumping source and pulse excitation of 3.3 W on the average is carried out to provide a pulse SHG laser output of 7.4 mW on the average. As stated above, the SHG laser output must be continuous waves. The required output is considered to be about 10 mW although it varies from one apparatus to another. A single-stripe type semiconductor laser which is capable of exciting the LiSAF laser and which is currently commercially available is of max. 500 mW class. The fundamental wave power is generally proportional to the exciting power in the input/output characteristic of a solid-state laser; therefore, the fundamental wave power is reduced approximately to one sixth when the pumping source is changed from the Kr laser to a semiconductor laser. In addition, the SHG output is proportional to the square of the fundamental wave power and therefore the expected SHG output will be about one thirty-sixth, i.e. a 0.2 mW pulse output. This indicates that no SHG output which is high enough for practical utilization will be accomplished unless the efficiency is improved greatly. Furthermore, the shape, i.e. the transverse mode of the output beam of a semiconductor laser is markedly flat in comparison with the Kr laser; therefore, it is anticipated that even if a beam shaping art is applied, the deterioration in the exciting efficiency cannot be prevented, with a resultant further reduction in the SHG output.
Hence, it is easily predicted that simply by changing the pumping source from the Kr laser to a semiconductor laser will not be able to achieve an SHG laser light source capable of providing output which is high enough for practical use. In addition, there is a concern in that even if higher output of the semiconductor laser is attained, the heat generated will increase and a forced-cooling means will be required, resulting in a large apparatus which heavily consumes electric power.
Thus, it has been found difficult to achieve an SHG laser, which employs the LiSAF laser excited by a semiconductor laser for solving all the problems involved in a gas laser, simply by combining the arts which have been disclosed. It is considered necessary, therefore, to achieve a small, low-electric power consumption SHG laser light source by identifying a cause of low efficiency and taking specific corrective measures and also to accomplish a laser-applied apparatus which employs the SHG laser light source.